The computer program breaks down each hexagonal mirror (facet) into 96 triangular elements. Tests are made for shadowing and blockage by elements of other mirrors. The relationship of the reflection of each element to the cavity absorber aperture is determined. Figure 4-6 shows the influence of geometric concentration ratio (ratio of projected concentrator area to aperture area) and facet count on the achieved concentration ratio (ratio of average flux through aperture to the ambient solar flux). Fig. 4-6. Solar Concentrator Performance Dashed lines in Figure 4-6 indicate the performance predictions made nearly two years ago by hand calculator. The performance data in Figure 4-6 does not include the effects of facet reflectivity (typically 0.88 for thin Kapton films with first surface oxidized aluminum) or gaps between the facets (perhaps 2% of their area). Figure 4-7 shows the influence on a reference solar concentrator of the sun being off-axis (not on the perpendicular to the center of the concentrator Fig. 4-7. Influences on Concentrator Efficiency (Solar Off-Axis Locations and Light Spread From Reflector Facets) dish) and of non-specular reflectivity from the plastic film reflectors. The sun can be off-axis either through SPS attitude control errors, or when flying "perpendicular to the orbit plane" at other than the equinoxes. Only a perfect mirror would yield a perfectly specular (parallel ray) reflection. Plastic film mirrors tend to produce scatter of the reflected rays. Figure 4-8 shows plastic film reflectivity data obtained by Boeing in a study for ERDA*. Testing was accomplished with a parallel beam (laser) light source. Masks of various diameters were used to assess the reflectivity versus cone angle. The cone angles shown do not include the 0.5° spread angle resulting from the angular width of the sun. The performance in Figure 4-6 includes the 0.5° solar width, and is for a light spread of 1° from the film. Kapton was baselined since some previous data (from project ABLE) indicated that Mylar was far more susceptible to radiation degradation. The rim angle yielding maximum efficiency (96.8%) was 40°. The 30° angle was kept as the baseline since it provides 96.0% efficiency with shorter cavity support arms. Fig. 4-8. Reflectivity Performance of Plastic Films Solar Reflector Susceptibility to Degradation in the Geosynchronous Environment—Damage to the solar concentrators by meteoroid particles has been assessed. The optical characteristics of the concentrators will be impaired by the scouring effect of small particles and by penetration of larger particles. All particles striking the concentrators will damage an area far greater than the cross section of the particle. The damage will consist of penetration, cratering and spallation. For the purpose of this assessment the particle specific gravity was assumed to be 0.05, and the diameter of the area damaged to be twenty times the particle diameter. *Contract E-(04-3)-IIII, "Central Receiver Solar Thermal Pov/er System."
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